Calculation of Vibrational Spectra of Linear Tetrapyrroles. 3. Hydrogen-Bonded Hexamethylpyrromethene Dimers Maria-Andrea Mroginski, †,‡ Ka ´ roly Ne ´ meth, † Tanja Bauschlicher, † Werner Klotzbu 1 cher, † Richard Goddard, § Oliver Heinemann, § Peter Hildebrandt,* ,‡ and Franz Mark* ,† Max-Volmer-Laboratorium, Institut fu ¨r Chemie, Technische UniVersita ¨t Berlin, Sekr. PC 14, Strasse des 17 Juni 135, D-10623 Berlin, Germany, Max-Planck-Institut fu ¨r Bioanorganische Chemie, Postfach 101365, D-45413 Mu ¨lheim an der Ruhr, Germany, and Max-Planck-Institut fu ¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu ¨lheim an der Ruhr, Germany ReceiVed: September 16, 2004; In Final Form: December 19, 2004 The structure and vibrational spectra of hexamethylpyrromethene (HMPM) have been investigated by X-ray crystallography, IR and Raman spectroscopies, and density functional theory calculations. HMPM crystallizes in the form of dimers, which are held together by bifurcated N-H(‚‚‚N) 2 hydrogen bonds, involving one intramolecular and one intermolecular N-H‚‚‚N interaction. The monomers are essentially planar, and the mean planes of the monomers lie approximately perpendicular to one another, so that the four N atoms in the dimer form a distorted tetrahedron. The structure of the HMPM dimer is well-reproduced by B3LYP/6-31G* calculations. A comparison of the calculated geometry of the dimer with that of the monomer reveals only small changes in the N-H‚‚‚N entity and the methine bridge angles upon dimerization. These are a result of weakening of the intramolecular N-H‚‚‚N hydrogen bond and the formation of a more linear N-H‚‚‚N intermolecular hydrogen bond. Using an empirical relation between the shift of the N-H stretching frequency of pyrrole and the enthalpy of adduct formation with bases [Nozari, M. S.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 7086-7090], estimates of the strength of the intra- and intermolecular hydrogen bonds are obtained. IR and Raman spectroscopies of HMPM and its isotopomers deuterated at the pyrrolic nitrogen atom and at the methine bridge reveal that the molecule is monomeric in nonpolar organic solvents but dimeric in a solid Ar matrix and in KBr pellets. The matrix IR spectra show a splitting of vibrational modes for the dimer, particularly those involving the N-H coordinates. Due to intrinsic deficiencies of the B3LYP/6-31G* approximation, a satisfactory reproduction of these modes of the monomeric and dimeric HMPM requires specific adjustments of the NH scaling factors for the calculated force constants and, in the case of the NH out-of-plane modes of HMPM dimers, also of intra- and intermolecular coupling constants. This parametrization does not significantly affect the other calculated modes, which in general reveal a very good agreement with the experimental data. Introduction Despite the methodological progress that has been achieved in NMR spectroscopy and X-ray crystallography in recent years, vibrational spectroscopy remains an indispensable tool for elucidating structure-dynamics-function relationships of co- factor-protein complexes, since it can provide important information about the hydrogen bond interactions that largely determine the molecular structure and reactivity of the prosthetic group. Continuing our previous studies on model compounds for the tetrapyrrole chromophore in the photoreceptor phyto- chrome (for recent reviews see refs 1-3), we address here hexamethylpyrromethene (HMPM; Chart 1). 4 The molecule is interesting for a number of reasons: First, it is a mimic for the central dipyrrole unit of phytochromobilin. 5 Second, it can form both intra- and intermolecular hydrogen bonds involving the N-H group, interactions which are thought to play a role in the chromophore-protein complex in phytochrome. Third, it represents an instructive model system for elucidating the effect of hydrogen bond interactions on the vibrational spectra of substituted pyrrole and pyrrolenine ring systems. In addition, early on in our investigations we realized that whereas HMPM occurs as a monomer in nonpolar organic solvents, in the solid it occurs as a dimer, enabling us to study in detail the effect of association on the vibration spectra. In a recent work, 4 we analyzed the resonance Raman (RR) spectrum of monomeric HMPM. However, Raman spectroscopy applied to this problem has the disadvantage that most of the modes that include coordinates of the N-H group exhibit only * Corresponding authors. (P.H.) Telephone: +493031421419. Fax: +0493031421122. E-mail: Hildebrandt@chem.tu-berlin.de. (F.M.) Tele- phone: +492083063697. Fax: +492083063951. E-mail: mark@ mpi-muelheim.mpg.de. † Max-Planck-Institut fu ¨r Bioanorganische Chemie. ‡ Technische Universita ¨t Berlin. § Max-Planck-Institut fu ¨r Kohlenforschung. CHART 1 2139 J. Phys. Chem. A 2005, 109, 2139-2150 10.1021/jp045805y CCC: $30.25 © 2005 American Chemical Society Published on Web 02/22/2005